1. Field of the Invention
This invention relates to an adjustable-period dual-tone multifrequency (DTMF) generator, and, particularly, to a generator for generating a DTMF signal using a digital circuit.
2. Description of the Prior Art
A conventional DTMF generator is typically an analog circuit device as shown in FIG. 1, in which block 1 is the pick-up switch assembly of DTMF voltage signal; blocks 2 and 3 form into an assembly of tracer and buffer circuits; block 4 is an analog signal adder; and block 5 is a reference voltage generator. The aforesaid analog circuit is subject to interference or signal distortions because of the manufacturing process, temperature or other environmental factors. Consequently, the output value thereof is often not conformed to the actual value. Furthermore, after an analog circuit has been used for a long time, the elements thereof are subject to aging, and the output values thereof will be affected. Therefore, the accuracy of a conventional analog circuit for generating DTMF signals is difficult to control, especially when operating at lower voltages.
Regarding the manufacturing process, the analog circuit has no flexibility in terms of circuit board area because of its characteristics, i.e., the circuit board can only be reduced to a limited extent, and cannot be reduced much further. Moreover, when the telephone line is designed, the column value signal (voltage value) and the row value signal have a ratio gain of 2 dB, but a conventional analog adder can only control gains in the range from 1-3 dB, i.e., it is unable to control the gain at exactly 2 dB, and therefore the accuracy thereof is not good.